1. Field of the Invention
The present invention relates to a flexible circuit board for mounting a light emitting element, an illumination apparatus, and a vehicle lighting apparatus. In particular, the present invention relates to a flexible circuit board for mounting a light emitting element that allows radiation of heat generated by a light emitting element, such as an LED, and efficient use of light emitted by the light emitting element, and an illumination apparatus and a vehicle lighting apparatus incorporating the same.
2. Description of the Related Art
A resin composition of, e.g., polyimide (PI), polyethylene terephthalate (PET), or polyethylene naphthalate (PEN) is generally used as the material for abase film of a flexible circuit board (FPC) (see Patent Documents 1 and 2). For example, Patent Document 2 discloses a configuration which uses polyimide or polyethylene terephthalate as the material for a base film of a flexible circuit board.
However, a configuration using a film of a resin composition as a base film of a flexible circuit board may suffer from the problems below.
Resin compositions as described above are expensive, which makes it difficult to reduce the price of a flexible circuit board.
Examples of a flexible circuit board include a carrier tape for TAB (Tape Automated Bonding). A carrier tape for TAB has a sprocket hole for positioning in bonding and a device hole for mounting a device, an electronic component, or the like. A sprocket hole and a device hole are openings extending through a base film in a thickness direction. Etching or die-punching is used to form such an opening. A process of forming an opening in a base film of a resin composition by etching is costly and is very low in processing rate. Die-punching requires facilities for punching, which increases facilities cost.
Flexible circuit boards have been increasing in width in order to, e.g., enhance production efficiency. However, a base film made of a resin composition is considerably deformed or varies widely in dimensions due to a temperature change, moisture absorption, or the like. For this reason, the yield of flexible circuit boards using a resin composition as the material for a base film tends to decrease with an increase in width.
In recent years, the density or the number of layers of wiring patterns to be formed at a flexible circuit board or devices or the like to be mounted on the flexible circuit board has been increasing. However, due to the low thermal conductivity of a resin composition, the higher density or the larger number of layers increases thermal load on a mounted device and the like. An increase in the density or the number of layers is thus hard to achieve.
The flexible circuit board configured as described above may pose the following problems when used as a substrate for mounting a light emitting element, such as a light emitting diode (LED). The LED used as a light emitting element deteriorates in characteristics at high temperatures and therefore needs to radiate heat in order to avoid becoming hot. In general, resin materials have low thermal conductivities, so that the flexible circuit board, whose base film and cover film are made of a resin material, may be unable to sufficiently radiate the heat generated by the LED. In addition, in order to efficiently use the light emitted by the LED, the amount of light absorbed by the surface of the flexible circuit board needs to be small. However, polyimide, which is used as the material of the base film and the cover film, is brown in color and therefore tends to absorb the light emitted by the LED, thereby reducing the amount of light available.
As a measure to radiate heat from a circuit board with an LED mounted thereon, for example, there is proposed an arrangement in which a metal radiator plate is disposed in close contact with the surface of the circuit board opposite to the surface on which the LED is mounted (see Patent Document 3). As another measure, there is proposed a flexible circuit board comprising a metal substrate and a wiring pattern formed on the metal substrate with an insulating film interposed therebetween (see Patent Document 4). As a measure to allow efficient use of light, an arrangement incorporating a cover film having a surface with a predetermined reflectance is proposed (see Patent Documents 4 and 5). However, the arrangement in which the radiator plate is mounted on the back surface leads to an increase in cost. Furthermore, none of the prior-art documents described above discloses an arrangement that achieves both radiation of the heat generated by the LED and efficient use of the light emitted by the LED.    Patent Document 1: Japanese Laid-open Patent Publication No. 05-029395    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-18926    Patent Document 3: Japanese Laid-Open Patent Publication No. 2009-25679    Patent Document 4: Japanese Laid-Open Patent Publication No. 2007-110010    Patent Document 5: Japanese Laid-Open Patent Publication No. 2009-302110    Patent Document 6: Japanese Laid-Open Patent Publication No. 2010-232252